Key controls on the formation of reservoirs along the ultra-deep strike-slip fault zone in the depression of the northern Tarim Basin, NW China

Based on the data of reservoir rock cores and 3D seismic inversion for reservoir, a comprehensive analysis was conducted using in-situ U-Pb dating of calcite cements, fluid inclusions, and geochemical data of fractured-vuggy reservoirs to investigate the key controls on the formation of reservoirs along the ultra-deep strike-slip fault zone in the depression, northern Tarim Basin, and establish the reservoir development model. The Middle Ordovician Yijianfang Formation contains tight matrix reservoirs and strike-slip faults with small displacement but relatively wide damage zone, forming a series of fault-fracture and fault-karst reservoirs which are distributed contiguously along the fault zone. Strike-slip faulting occurred during the deposition of the Yijianfang Formation, giving rise to penecontemporaneous atmospheric freshwater dissolved pores/vugs. The U-Pb ages of 440–468 Ma obtained from calcite cements in the fractures/vugs indicate that the reservoirs along the strike-slip fault zone were formed in Middle to Late Ordovician. Data of reservoir fluid inclusions, trace elements, and C/O/Sr isotopic compositions suggest that the fracture/vug cementation and filling took place in a penecontemporaneous to shallow burial stages dominated by atmospheric freshwater. On the basis of intra-platform high-energy shoal deposits, strike-slip faulting coupled with dissolution is identified as the primary control on reservoir formation and spatial distribution, and a penecontemporaneous–shallow burial strike-slip fault-controlled reservoir development model is thus proposed. Comprehensive analysis indicates that large-scale fault-fracture and fault-karst reservoirs can develop along ultra-deep strike-slip fault zone in intracratonic depression, with their scales and distribution scope controlled by the coupling of facies, faulting, and dissolution processes in the penecontemporaneous–shallow burial stages.